1. Field of the Invention
The present invention relates to a portable information device to transmit and receive plural kinds of wireless signals.
2. Description of the Related Art
With the rapid development of communication technologies a wireless system becomes explosively widespread, which is utilized for portable information devices such as a notebook computer and a cellular phone. As typical examples of such a wireless system, there are given: a local area network technology, MIMO (Multiple Input Multiple Output), for transmitting and receiving data with a plurality of antennas; Bluetooth for communicating between information devices; metropolitan area network technologies, IEEE802.16/IEEE802.16/IEEE802.20, for covering the area of about 50 km in radius with a single antenna using a frequency band of 2-66 GHz; and UWB (Ultra Wide Band) for transmitting and receiving signals with an extremely wide frequency band with about 1 GHz.
When a wireless system is mounted on a notebook computer as an example of a mobile device, an antenna of the wireless system is arranged at an upper portion of a display of the notebook computer to easily receive radio waves and a transmitting and receiving circuit of the wireless system is arranged in a main body of the notebook computer. This arrangement makes a path (mainly a cable) long, which connects the antenna and the transmitting and receiving circuit. Since such a long cable serves as a low-pass filter (LPF), the cable attenuates a high frequency signal with a gigahertz frequency band, which is expected to be used in portable information devices. Thus, the carrier to noise ratio (CN ratio) of reception signals becomes small and a noise figure (NF) becomes worse in the main body. Here the noise figure is an index that shows the ratio of ‘the CN ratio of an input signal’ to ‘the CN ratio of an output signal’.
Japanese Patent Publication Laid-open No. 2006-33076 employs a technique, called “an active antenna”, that provides a reception line and a transmission line near an antenna and arranges an amplifier on the reception line for suppressing the NF degradation.
FIG. 1 shows a schematic configuration of a wireless device recited in the above patent document. The wireless device comprises an active antenna 1, a transmitting and receiving device 2, a signal line 3 that connects the active antenna 1 and the transmitting and receiving device 2, and a transmission control signal detecting unit 6. When a signal is transmitted, the transmission control signal detecting unit 6 generates a control voltage for switches 13, 14 using transmission power on the signal line 3. When a signal is received, an amplifier 12 is activated and the switches 13, 14 are controlled using a voltage superimposed in the signal on the signal line 3. This wireless device allows us to use only a single signal line for both of reception and transmission.
With the active antenna 1, the amplifier 12 amplifies a signal received with an antenna 11 before attenuated by passing through the signal line 3. Accordingly, even though the received signal attenuates through the signal line 3, the reduction of the CN ratio can be suppressed and consequently the degradation of the NF of the wireless device can be prevented.